The invention relates to a system for measuring the volume value of a confined space, in particular the functional residual capacity of the lungs of a patient, by means of a measuring space having a variable volume and a gas analyzer, which measuring space can be connected to the confined space and is provided with means for supplying and removing air, as well as for supplying oxygen and a detectable gas, the gas analyzer generating an output signal which is a linear function of the concentration of this detectable gas and the zero level of which is adjustable.
With prior-art systems of this kind, it is necessary first to determine the so-called dead space of the measuring space before the volume value of the confined space can be measured, while the required volume is only available on the basis of a calculation. This makes the use of prior-art systems relatively awkward.